Using rivets as mechanical fasteners to join components (e.g., sheets of material) is widespread in many industries. In recent years, polymer based composite materials have been widely used within applications for purposes such as decreasing weight components and fasteners.
Fast, easy-to-use rivets allow for relatively speedy assembly, consistent mechanical performance and streamlined installed appearance, making riveting a reliable and economical assembly method in areas such as assembly of bridge components and joining of automotive workpieces. Rivets are regularly used with similar dissimilar metals such as combinations amongst aluminum, stainless steel, and copper, among others.
Before being installed, a rivet body (e.g., a blind rivet) consists of a rivet head and a smooth cylindrical rivet shank. The rivet body is set using a mandrel, driven through the rivet body. Upon installation, the rivet body is placed into an installation tool and then inserted into a punched or pre-drilled hole within the workpiece. Activating an installation tool pulls the mandrel into the rivet body and securely clamps the workpieces together. When the mandrel reaches a predetermined break-load, the mandrel breaks away and is removed from the set rivet body. A small portion of the mandrel remains in the bottom of the rivet shank to ensure the clamping force is retained in the joint. In final form, the rivet shank is deformed (known as upsetting), so that the rivet shank expands larger than the original rivet shank diameter, thus holding the rivet body in place.
Using pre-drilled holes for riveting can cause future issues with the integrity of the joint. Problems associated with pre-drilled holes include issues such as, but not limited to, drilling with the wrong size drill bit, drilling of the intended pre-drilled hole center mark, and forgetting to deburr holes in the workpieces, which can create debris within the hole and thus the joint. Additionally, in situations where flush riveting is necessary, forgetting to provide a top and/or bottom dimple or making a machined-countersunk hole too shallow or too deep can cause future problems with the joint. Moreover, the need for pre-drilled holes can add significant cost and time to a manufacturing process.
Additionally, the use of pre-drilled holes can increase possible galvanic corrosion of the workpieces. When dissimilar workpieces come into contact with one another in the presence of an electrolyte (e.g., water), an electrochemical process known as a galvanic reaction occurs. The galvanic reaction, known as galvanic corrosion, corrodes one metal at a faster rate and the other more slowly. The rate of corrosion depends upon a) the difference in electrical potential of the conductors, b) the conductivity of the electrolyte and c) the relative sizes of the contacting areas.
Joining composite workpieces, especially an electrically conductive composite workpiece, with rivets require special efforts to minimize or prevent the galvanic corrosion. Often galvanic corrosion can be associated with noise generated by contact and sealing of the joined workpieces. The corrosion on the surface area where the rivet joins the workpieces can cause noise (e.g., a grinding or squeaking) and issues with sealing the workpieces. Additionally, in environments high in moisture content (e.g., air conditioners) galvanic corrosion can cause pre-mature break down at the joined location of the workpieces (i.e., at the rivet), causing possible failure of the joint.
Previous attempts have been tried to reduce and/or avoid galvanic corrosion, where corrosion becomes a threat to the serviceability of the joint, for example, incorporating a barrier between the workpieces. Barriers may include painted added to one or more portions of the workpieces, or washers and/or gaskets inserted between the workpieces to prevent direct contact. Adding a protective coating (e.g., anodic oxide finish) to the rivet shank has also been used in an effort to prevent galvanic corrosion.
Although barriers and coatings provide a corrosion-resistant finish, barriers and coatings increase the surface of the rivet shank because the barrier/coating is placed on the outer surface of the rivet body, which occupies more space than a base metal of the rivet body. An increased surface may create a potential issue of undesirably low machine tolerances or assembly tolerances. As an example, in the case of small holes threaded to accept a fastener (e.g., used when joining surfaces in aircraft or automotive industries), an outer surface coating may cause the rivet shank to bind within the threads of the hole. Moreover, when barriers and coatings are used, it is difficult to ensure that upon the insertion of the rivet the integrity of the barriers and coatings are maintained (e.g., not scratched). Scratches of the barriers and coatings may be created upon rivet insertion and form reactive sites for galvanic corrosion, thereby causing risks for noise and seal issues at the joint and premature failure of the joint.